Time Series Graph

I am trying to create a graph for real estate needs that reflects square footage (y axis) to upcoming lease expirations (x axis). Yet, when I do so, the time series graph only shows one value per year.

For instance, I might have three leases expiring in 2012 for a total square footage of 20,000. I want the graph to show this 20,000 square feet, but instead it only shows one of the three expiring leases for 10,000.

It seems the data overlaps on this bar graph. Is there any way to fix this in excel 2003 or 2007??